There has been a general trend toward in-line color picture tubes with greater deflection angles in order to provide shorter tubes. In a tube with 110.degree. deflection, it has been found that the electron beams become excessively distorted as they are scanned toward the outer portions of the screen. Such distortions are commonly referred to as flare and appear on the screen of the tube as an undesirable low intensity tail or smear extending from a desirable intense core or spot. Such flare distortions are due, at least in part, to the effects of the fringe portions of the deflection field of the yoke on the beam as it passes through the electron gun, and to the nonuniformities in the yoke deflection field itself.
When the yoke's fringe field extends into the region of the electron gun, as is usually the case, the beams may be deflected slightly off axis and into a more aberrated portion of an electron lens of the gun. The result is frequently a flare distortion of the electron beam spot which extends from the spot toward the center of the screen. This condition is particularly troublesome in self-converging yokes having a toroidal deflection coil, because of the relatively strong fringing of toroidal type coils.
Self-converging yokes are designed to have a nonuniform field in order to increasingly diverge the beams as the horizontal deflection angle increases. This nonuniformity also causes vertical convergence of the electrons within each individual beam. Thus, the beam spots are vertically overconverged at points horizontally displaced from the center of the screen, causing a vertically extending flare both above and below the beam spot.
The vertical flare due to both the effects of the yoke's fringe field in the region of the gun and to the nonuniform character of the yoke field itself is an undesirable condition which contributes to poor resolution of a displayed image on the screen.
It is known to provide non-symmetrical electron gun electrodes to in turn provide a desired astigmatism in the electron optics of the gun for the purpose of compensating for the above-described flare astigmatism. An example of this is disclosed in U.S. Pat. No. 4,234,814 issued to Chen and Hughes on Nov. 18, 1980. This Patent describes a screen grid electrode having an aperture which is of rectangular cross-section facing backward toward the cathode and circular cross-section facing forward toward the screen. This astigmatic screen grid is of one piece, electrically speaking, and is energized during tube operation with a fixed DC bias voltage. While this gun does indeed reduce flare astigmatism to a degree sufficient for some tubes, still further correction is desirable for other tubes, particularly very wide angle deflection tubes and particularly where they are to be used to display printed matter near the corners of the screen.